This invention relates to a magnetic recording medium such as video tape, audio tape, computer data tape, etc. More particularly, the invention relates to such a medium with a novel binder which is curable by electron beam radiation.
Magnetic recording media currently used in various recording equipment use, as a binder, thermoplastic resins, such as vinyl chloride-vinyl acetate type resin, vinyl chloride-vinylidene chloride type resin, cullulosic resin, acetal resin, urethane resin, acrylonitrile butadiene resin, etc., alone or in combination as described in, for example, U.S. Pat. Nos. 3,634,137, 4,238,548 and 4,307,154. However, the use of these thermoplastic resins as binders has a disadvantage that wear resistance of a magnetic layer is poor to the extent that it can contaminate the tape guiding systems during tape running.
Further, a process of using a thermosetting resin such as melamine resins or urea resins or a process of adding a binder capable of crosslinking upon chemical reaction, such as an isocyanate compound and an epoxy compound, to the above-described thermosetting resin is also known as described in, for example, U.S. Pat. Nos. 4,049,871, 4,154,895 and 4,333,988. However, the use of the crosslinkable binder deteriorates storage stability of a resin solution in which magnetic powders are dispersed, i.e., shortens the pot life of the resin solution. In other words, the magnetic coating composition cannot be maintained uniform in property, resulting in poor uniformity of the magnetic tape. Further, after coating and drying the magnetic coating composition, heat treatment is necessary for curing a coating layer thereby requiring a long period of time for the production of magnetic recording media.
In order to eliminate the above-described disadvantages, a method has been proposed wherein a magnetic recording medium is produced by using a combination of an acrylate oligomer and an acrylate type monomer as a binder and curing the binder by electron beam radiation as disclosed in Japanese Patent Publication No. 12423/72 and Japanese Patent Application (OPI) (Open to Public Inspection) Nos. 13639/72, 15104/72 and 77433/75. Further, a technique of using a binder comprising a thermoplastic resin and an electron beam-curing resin as described in, for example, U.S. Pat. Nos. 3,871,908, 4,004,997 and 4,343,861 is disclosed in Japanese Patent Application (OPI) Nos. 25231/81, 86130/82, 86131/82 and 127926/82. These improved processes, however, failed to produce magnetic recording media having high electromagnetic properties and running durability.
Recently, it has been desired that a support for magnetic tapes be made thinner for achieving long-time recording and that the dynamic properties of the magnetic layer be improved. Further, an improvement in dispersibility of a magnetic powder has been required to attain high density recording. However, the above-described conventional techniques could not impart the properties that satisfy the above requirements for a magnetic layer.
The inventors have extensively investigated to eliminate the disadvantages associated with the conventional techniques such as a process of using a thermoplastic resin or a thermosetting resin, a process of adding a binder crosslinkable by chemical reaction, or a process of using a binder capable of hardening upon electron beam crosslinking and, as a result, reached the present invention.